When a soft magnetic steel material forming a component of a magnetic circuit is employed in a DC magnetic field, and even when in is employed in an AC magnetic field of which frequency is lower than a commercially available frequency, its core-loss property, which is one of the items for its evaluation on AC properties, is of little importance, but it is rather desirable that its coercive force, which is one of the items for its evaluation on DC magnetization properties, be small to, for example, reduce the residual magnetism in the component of the magnetic circuit and ensure the linearity of its performance. The material is also desired to have a high magnetic flux density to provide an efficiently working component of a magnetic circuit.
The arts aimed at the solution of those problems are known from, for example, Japanese Patent Applications laid open under Nos. Hei 3-75314 and Hei 3-20447. They are all intended for improving the DC magnetization properties of soft magnetic steel materials based on pure iron. Those materials have a good level of magnetic flux density owing to the inherently high saturation magnetization of iron, and include some having a low coercive force, too.
None of those arts, however, discloses any measure for ensuring the corrosion resistance of materials. It, therefore, follows that, when any component of a magnetic circuit based on those arts is used for any application calling for corrosion resistance, it is essential to give it surface treatment, such as plating or coating. The addition of a large amount of chromium to steel for imparting to it corrosion resistance comparable to that of stainless steel is known from, for example, Japanese Patent Applications laid open under Nos. Hei 3-150313 and Hei 2-259047. Although it is necessary to add expensive chromium in the amount of 5 to 8% by weight, or even more to impart corrosion resistance to steel, Hei 3-150313 shows by way of an example that an excellent coercive force can be obtained. As regards magnetic flux density, however, the addition of chromium has the disadvantage of bringing about a lower magnetic flux density, as is typically shown by Hei 2-259047. The above proposal specifies a minimum magnetic flux density of 11,000 G.
The formation of an oxide film on the surface of a steel material is known from, for example, Japanese Patent Application laid open under No. Hei 1-283343. These arts are, however, not intended for improving the corrosion resistance of the material, but are intended for improving its core-loss property as one of the aspects of its AC properties, or for preventing the formation of any inner oxide layer in the material during its annealing.
As is obvious from the foregoing, the known materials based on iron do not have a satisfactorily low coercive force, though they have a good level of magnetic flux density. Moreover, as there has not been developed any art of manufacturing materials which are resistant to corrosion by themselves, the surface treatment, such as plating or coating, of the materials is necessary for imparting corrosion resistance to them, and adds to the manufacturing cost of components of a magnetic circuit. The known materials based on stainless steel, and having a greatly improved corrosion resistance have the drawback of necessitating the addition of a large amount of chromium which is expensive, and yet brings about an unavoidable lowering in their magnetic flux density.